In an LTE-Advanced (Long Term Evolution-Advanced) system, CA (Carrier Aggregation) is under discussion to achieve higher throughput using a plurality of CCs (Component Carriers). According to CA, a primary cell (PCell: Primary Cell) and a secondary cell (SCell: Secondary Cell) are provided. The primary cell is reliable and used to maintain connectivity with a mobile station (UE: User Equipment). The secondary cell is additionally configured for a mobile station connecting to the primary cell.
According to CA, not only a plurality of CCs belonging to the same base station but also CCs belonging to different base stations can be used. Communication using CCs belonging to different base stations is referred to as “Dual Connectivity” (see 3GPP TR 36.842 V12.0.0 (2013-12)). In Dual Connectivity, a base station corresponding to a PCell is referred to as an “MeNB (Master eNB)” and a base station corresponding to an SCell is referred to as an “SeNB (Secondary eNB)”. Dual Connectivity is useful when all CCs cannot be accommodated in a single base station or when higher throughput needs to be achieved in an area where an ideal backhaul cannot be adequately installed.